Method for producing a treated natural gas, a cut rich in c3+ hydrocarbons and optionally an ethane-rich stream, and associated facility

ABSTRACT

The method includes the following steps, sampling a recycling stream ( 152 ) in a head stream ( 131, 140, 141 ) stemming from a recovery column ( 35 ), establishing a heat exchange relationship of the recycling stream ( 152 ) with at least one portion of the head stream ( 131 ) stemming from the recovery column ( 35 ), reintroducing, after expansion, the cooled and expanded recycling stream into the recovery column ( 35 ). The method includes sampling in the bottom of the recovery column ( 35 ) of at least one bottom reboiling stream ( 165 ), and establishing a heat exchange relationship of the re-boiling stream with at least one portion of the initial natural gas ( 13 ) or/and with the recycling stream ( 152 ), the bottom reboiling being ensured by the calories taken from the initial natural gas stream ( 13 ) or/and from the recycling stream ( 152 )

The present invention relates to a method for simultaneously producing atreated natural gas, a cut rich in C₃ ⁺ hydrocarbons, and under at leastcertain conditions of production, an ethane-rich stream, from an initialnatural gas stream containing methane, ethane and C₃ ⁺ hydrocarbons, themethod comprising the following steps:

-   -   cooling and partial condensation of the initial natural gas        stream in at least one upstream heat exchanger in order to form        a cooled initial stream;    -   separating the cooled initial gas stream into a liquid flow and        into a gas flow;    -   expanding the liquid flow, and introducing a stream from the        liquid flow into a column for recovering C₂ ⁺ hydrocarbons at a        first intermediate level;    -   forming a turbine feed stream from the gas flow;    -   expanding the feed stream in a dynamic expansion turbine and        introducing it into the recovery column at a second intermediate        level;    -   recovering and compressing at least one portion of the head        stream of the recovery column in order to form the natural gas        and recovering the foot stream of the recovery column in order        to form a liquid stream rich in C₂ ⁺ hydrocarbons;    -   introducing the liquid stream to a feed level of a fractionation        column provided with a head condenser, the ethane-rich stream        being produced, under said production conditions, from a stream        stemming from the fractionation column, the fractionation column        producing a foot stream intended to form at least partly the C₃        ⁺ hydrocarbon cut;    -   introducing a primary reflux stream produced in the head        condenser with reflux into the fractionation column;    -   producing a secondary reflux stream from the head condenser and        introducing the secondary reflux stream at the head of the        recovery column.

Such a method is intended for treating a natural gas stream in order toextract at least the C₃ ⁺ hydrocarbons therefrom, in order to recoverliquids from the natural gas and an adjustable amount of C₂hydrocarbons.

The C₂ and C₃ ⁺ hydrocarbons are extracted from the initial natural gasin order to avoid condensation during the transport or/and the handlingof the gas. This condensation may lead to the production of liquid plugsin the transport facilities, which is detrimental to production.Further, these hydrocarbons may be marketed with significant merchantvalue, which contributes to the cost effectiveness of the facilities.

Subsequently, methods have been developed for simultaneously extractingalmost all the C₃ ⁺ hydrocarbons present in the initial natural gas, anda high proportion of the ethane present in the initial gas.

However, the demand for ethane on the market is highly fluctuating,while that for C₃ ⁺ hydrocarbon cuts is relatively constant and is ofconsiderable value.

In certain cases, it is therefore necessary to reduce the production ofethane in the method, by reducing the extraction rate of this compoundin the recovery column. In this case, the extraction rate of C₃ ⁺hydrocarbons also decreases, which reduces the cost effectiveness of thefacility.

In order to overcome this problem, it is known how to provide doublefacilities, i.e. comprising a secondary unit optimized for producing C₃⁺ hydrocarbons when ethane extraction is nil. Such a secondary unit isexpensive to operate and to maintain.

Patent U.S. Pat. No. 7,458,232 discloses a solution to this problem, byproposing a method which guarantees optimized extraction of C₃ ⁺hydrocarbons, generally of more than 99%, and which nevertheless attainsflexible ethane recoveries comprised, for example, between 2% and 85%,depending on the composition of the load gas.

The method described in U.S. Pat. No. 7,458,232 is thereforeparticularly effective and remains highly flexible. However, when theethane extraction rate increases, energy consumption resulting from theuse of compressors also increases. An improvement in the yield of thefacility, notably for high ethane recovery rates, is therefore alwaysdesirable.

An object of the invention is to obtain a method with which it ispossible to obtain in a flexible way ethane extraction rates which mayrange up to 85%, while notably reducing the energy consumption of thefacility.

For this purpose, the object of the invention is an insulation of theaforementioned type, characterized in that the method includes thefollowing steps:

-   -   sampling a recycling stream in the head stream stemming from the        recovery column;    -   establishing a heat exchange relationship of the recycling        stream with at least one portion of the head stream stemming        from the recovery column,    -   reintroducing, after expansion, the cooled and expanded        recycling stream into the recovery column;

the method including the sampling in the bottom of the recovery columnof at least one bottom reboiling stream, and the establishment of a heatexchange relationship of the bottom reboiling stream with at least oneportion of the initial natural gas or/and with the recycling stream, thebottom reboiling being ensured by the calories taken from the initialnatural gas stream or/and from the recycling stream.

The method according to the invention may comprise one or more of thefollowing features, taken individually or according to all technicallypossible combinations:

-   -   at least one portion of the head stream of the recovery column        and the recycling stream are placed in a heat exchange        relationship with the initial natural gas stream and with the        bottom reboiling stream;    -   the recycling stream stemming from the first upstream heat        exchanger, the secondary reflux stream stemming from the head        condenser and the head stream stemming from the recovery column        are put into a heat exchange relationship in a first head heat        exchanger;    -   at least one side reboiling stream is sampled above the bottom        reboiling stream, said or each side reboiling stream being        placed in a heat exchange relationship with at least one portion        of the initial natural gas stream;    -   the ethane-rich current is drawn off from an intermediate level        of the fractionation column located above the level for feeding        the column, and below the head level of the fractionation        column;    -   it includes the following steps:        -   separating the initial natural gas stream into a first            initial stream and a second initial stream;        -   introducing the first initial stream into the first upstream            heat exchanger;        -   introducing at least one portion of the second initial            stream into an auxiliary dynamic expansion turbine in order            to form an auxiliary reflux stream from the effluent            stemming from the auxiliary turbine;        -   introducing the auxiliary reflux stream into the recovery            column;    -   at least one portion of the recycling stream is compressed in an        auxiliary compressor coupled with the auxiliary turbine;    -   at least one portion of the head stream is compressed in an        auxiliary compressor coupled with the auxiliary turbine,        advantageously between a first compressor coupled with the first        turbine and a second compressor,    -   it includes a step for compressing at least one portion of the        head current in a first compressor coupled with the first        turbine, and then a step for compressing the partly compressed        head stream in a second compressor, the recycling stream being        sampled downstream from the second compressor.    -   at least one secondary recycling stream is sampled in the        recycling stream, the secondary recycling stream being        introduced into a secondary expansion turbine before being        reintroduced into the head stream, advantageously upstream from        a passage of the head stream in the first upstream heat        exchanger;    -   the secondary reflux stream consists of a liquid, of a gas, or        of a liquid and gas mixture stemming from the head condenser of        the fractionation column;    -   it includes the sampling, in the recycling stream, of a bypass        stream, the bypass stream being reintroduced into a stream        located upstream from the first dynamic expansion turbine;    -   the liquid flow stemming from the first upstream separator flask        is expanded and is introduced into a second upstream separator        flask in order to form a liquid fraction and a gas fraction,    -   the liquid fraction being introduced after expansion at the        first intermediate level of the recovery column, the gas        fraction being introduced at an upper level of the recovery        column, located below the intermediate level,    -   the liquid flow stemming from the first upstream separator flask        being advantageously placed in a heat exchange relationship with        the initial natural gas stream in order to be heated up before        being introduced into the second upstream separator flask;    -   it includes the establishment of a heat exchange relationship of        the foot stream stemming from the recovery column with the        initial natural gas stream and with the bottom reboiling stream        in the first upstream heat exchanger before its introduction        into the fractionation column;    -   the gas flow stemming from the first separator flask is        separated into the feed stream and into a reflux stream, the        feed stream being intended for feeding the dynamic expansion        turbine, the reflux stream being introduced, after cooling,        partial or total condensation and expansion in a valve, with        reflux, into the recovery column;    -   it includes a step for compressing the foot stream stemming from        the recovery column in a pump, before its introduction into the        fractionation column.    -   the method includes a step for cooling the secondary reflux        stream by heat exchange with at least one portion of the head        stream of the recovery column.

The object of the invention is also a facility for simultaneousproduction of a treated natural gas, of a cut rich in C₃ ⁺ hydrocarbons,and under at least certain conditions of production, an ethane-richstream from an initial natural gas stream containing methane, ethane,and C₃ ⁺ hydrocarbons, the facility comprising:

-   -   an assembly for cooling and partly condensing the initial        natural gas stream comprising at least one first upstream heat        exchanger in order to form a cooled initial stream;    -   an assembly for separating the cooled initial current into a        liquid flow and a gas flow;    -   a column for recovering C₂ ⁺ hydrocarbons    -   an assembly for expansion of the liquid flow, and for        introducing a stream stemming from the liquid flow into the        recovery column at a first intermediate level;    -   an assembly for forming a stream for feeding a turbine from the        gas flow;    -   an assembly for expansion of the feed stream, comprising a        dynamic expansion turbine and an assembly for introducing the        expanded feed stream into the recovery column at a second        intermediate level;    -   an assembly for recovering and compressing at least one portion        of the head stream of the recovery column in order to form        natural gas and an assembly for recovering the foot stream of        the recovery column in order to form a liquid stream rich in C₂        ⁺ hydrocarbons;    -   a fractionation column provided with a head condenser,    -   an assembly for introducing the liquid stream at a feed level of        the fractionation column, the ethane-rich stream being able to        be produced, under said production conditions, from a stream        stemming from the fractionation column, the fractionation column        being able to produce a foot stream intended to form at least        partly the C₃ ⁺ hydrocarbon cut;    -   an assembly for introducing a primary reflux stream produced in        the head condenser, with reflux, into the fractionation column;    -   an assembly for producing a secondary reflux stream from the        head condenser and an assembly for introducing the secondary        reflux stream at the head of the recovery column,

characterized in that the facility includes:

-   -   an assembly for sampling a recycling stream in the head stream        of the recovery column;    -   an assembly for establishing a heat exchange relationship of the        recycling stream with at least one portion of the head stream        stemming from the recovery column,    -   an assembly for reintroducing, after expansion, the recycling        stream into the recovery column, the facility further including        an assembly for sampling in the bottom of the recovery column at        least one bottom reboiling stream and an assembly for        establishing a heat exchange relationship of the bottom        reboiling stream with at least one portion of the initial        natural gas or/and with the recycling stream, reboiling being        able to be ensured by the calories taken in the initial natural        gas stream or/and in the recycling stream.

The facility according to the invention may comprise one or more of thefollowing features, taken individually or according to all technicallypossible combinations:

-   -   it includes a first upstream heat exchanger capable of        establishing a heat exchange relationship with at least one        portion of the initial natural gas stream, the bottom reboiling        stream, optionally side reboiling streams, at least one portion        of the head stream and the recycling stream;    -   it includes a first upstream heat exchanger capable of        establishing a heat exchange relationship with a first portion        of the initial natural gas stream, with at least one portion of        the head stream, a second upstream heat exchanger, distinct from        the first upstream heat exchanger, capable of establishing a        heat exchange relationship of a second portion of the initial        gas stream with the bottom reboiling stream stemming from the        recovery column, and a third upstream heat exchanger distinct        from the first upstream heat exchanger and from the second        upstream heat exchanger, the third upstream heat exchanger being        capable of establishing a heat exchange relationship of at least        one portion of the recycling stream with at least one portion of        the head stream, the facility advantageously including an        auxiliary compressor capable of compressing the portion of the        recycling stream intended to be introduced into the third        upstream heat exchanger;    -   the facility comprises a first head heat exchanger, capable of        placing in a heat exchange relationship at least one portion of        the head stream, optionally with the reflux stream, and the        secondary reflux stream;    -   the facility comprises a second head heat exchanger, distinct        from the first head heat exchanger and capable of establishing a        heat exchange relationship between a second portion of the head        stream and the recycling stream.

The invention will be better understood upon reading the descriptionwhich follows, only given as an example, and made with reference to theappended drawings, wherein:

FIG. 1 is a functional block diagram of a first facility for applying afirst method according to the invention,

FIG. 2 is a diagram similar to FIG. 1 of a second facility for applyinga second method according to the invention;

FIG. 3 is a diagram similar to FIG. 1 of a third facility for applying athird method according to the invention;

FIG. 4 is a diagram similar to FIG. 1 of a fourth facility for applyinga fourth method according to the invention;

FIG. 5 is a diagram similar to FIG. 1 of a fifth facility for applying afifth method according to the invention;

FIG. 6 is a diagram similar to FIG. 1 of a sixth facility, for applyinga sixth method according to the invention, the sixth facility resultingfrom de-bottlenecking of an existing facility.

The first facility 11 according to the invention, illustrated in FIG. 1,is intended for simultaneously producing from an initial desulfurized,dry and at least partly decarbonated natural gas stream 13, a treatednatural gas 15 as a main product, a cut 17 of C₃ ⁺ hydrocarbons and anethane-rich stream 19 with adjustable flow rate.

The term of “at least partly decarbonated” means that the carbon dioxidecontent in the initial natural gas stream 13 is advantageously less thanor equal to 50 ppm when the treated natural gas 15 has to be liquefied.This content is advantageously less than 3% when the treated natural gas15 is directly sent to a gas distribution network.

Also, the water content is less than 1 ppm, advantageously less than 0.1ppm.

The facility 11 comprises a unit 21 for recovering C₂ ⁺ hydrocarbons anda unit 23 for fractionation of C₂ ⁺ hydrocarbons.

In all of the following, a liquid flow and the conduit which conveys it,will be designated by a same reference, the relevant pressures areabsolute pressures and the relevant percentages are molar percentages.

The unit 21 for recovering C₂ ⁺ hydrocarbons successively comprises afirst upstream heat exchanger 25, a first upstream separator flask 27, afirst upstream turbine 29, coupled with a first compressor 31, a firsthead heat exchanger 33, and a recovery column 35 provided with at leastone side reboiling circuit 37, 39 and with a side reboiling circuit 41.

In this example, the column 35 is provided with two side reboilingcircuits 37, 39.

The unit 21 further comprises a second compressor 43 driven by anexternal energy source and a first cooler 45 placed downstream from thesecond compressor 43. The unit 21 also comprises a column bottom pump47.

The fractionation unit 23 comprises a fractionation column 61. Thecolumn 61 includes at its head, a head condenser 63 and at its foot, areboiler 65.

The head condenser 63 comprises a second cooler 67 and a firstdownstream separator flask 69 associated with a reflux pump 71.

A first method according to the invention applied by means of thefacility 11 will now be described.

An exemplary initial molar composition of the initial desulfurized, dryand at least partly decarbonated natural gas stream 13 is given in thetable below.

Molar fraction in % Helium 0.0713 CO₂ 0.0050 Nitrogen 1.2022 Methane85.7828 Ethane 10.3815 Propane 2.1904 i-butane 0.1426 n-butane 0.1936i-pentane 0.0204 n-pentane 0.0102 Hexane 0.0000 Total 100.0000

More generally, the molar methane fraction in the initial natural gasstream 13 is comprised between 75% and 90%, the molar fraction of C₂ ⁺hydrocarbons is comprised between 5% and 15%, and the molar fraction ofC₃ ⁺ hydrocarbons is comprised between 1% and 8%.

The load flow rate to be treated for example is of the order of 38,000kmol/h. The initial natural gas stream 13 has a temperature close toroom temperature and notably substantially equal to 20° C., and apressure notably greater than 35 bars.

In a particular example, the natural gas stream 13 has a temperature of20° C. and a pressure of 50 bars absolute.

In the facility illustrated in FIG. 1, the initial natural gas stream 13is cooled and at least partly condensed in the first upstream heatexchanger 25 in order to form a cooled initial stream 113.

The cooled initial stream 113 is introduced into the first upstreamseparator flask 27 in which a separation is performed between a gasphase 115 and a liquid phase 117.

The liquid phase 117 forms, after passing into an expansion valve 119,an expanded mixed phase 120 which is introduced at a first intermediatelevel N1 of the recovery column 35, located in the upper region of thecolumn, above the side reboiling circuits 37 and 39.

By “intermediate level” is meant a location including distillation meansabove and below this level.

The gas fraction 115 is separated into a feed stream 121 and a refluxstream 123.

Advantageously, the molar flow rate of the feed stream 121 is greaterthan the molar flow rate of the reflux stream 123.

The feed stream 121 is expanded in the turbine 29 down to a pressureclose to that of the column 35 in order to obtain an expanded feedstream 125. The stream 125 is introduced into the recovery column 35 ata second intermediate level N2, located above the first intermediatelevel N1.

The reflux stream 123 is partly or totally condensed in the first headheat exchanger 33, and is then expanded in an expansion valve 127 inorder to form an expanded reflux stream 128. This stream 128 isintroduced into the recovery column 35 at a third intermediate level N3,located above the intermediate level N2.

The pressure of the recovery column 35 is for example comprised between12 and 40 bars.

The recovery column 35 produces a head stream 131 which is heated up inthe first head heat exchanger 33 by heat exchange with the reflux stream123 in order to form a partly heated-up head stream 139.

The stream 139 is again heated up in the first upstream heat exchanger25 by heat exchange with the initial natural gas stream 13 in order toform a heated-up head stream 140.

The heated-up head stream 140 is then compressed in the first compressor31, and then in the second compressor 43, in order to form a compressedhead stream 141. The pressure of the stream 141 is greater than 25 bars,for example equal to 50 bars. The stream 141 is then cooled in the firstcooler 45 in order to form the treated natural gas 15.

According to the invention, a recycling stream 152 is sampled in thehead stream stemming from the column 35. In the example illustrated inFIG. 1, the recycling stream 152 is sampled in the compressed heated-uphead stream 141, after its cooling in the first cooler 45.

The ratio of the molar flow rate of the recycling stream 152, relativelyto the molar flow rate of the head stream 131 stemming from the recoverycolumn 35 is comprised between 0% and 25%.

The recycling stream 152 is then introduced into the first upstream heatexchanger 25 so as to be cooled therein by heat exchange with at leastone portion of the head stream 131. In this example, the stream 152 isplaced in a heat exchange relationship with the partly heated-up headstream 139 stemming from the head heat exchanger 33, in order to form apartly cooled recycling stream 154.

The stream 154 is then introduced into the head heat exchanger 33, inorder to be cooled therein by heat exchange with the head stream 131,and to form after expansion in a valve 156, a cooled recycling stream155.

The cooled recycling stream 155 is introduced into the recovery column35 at a level N5 located above the level N3, advantageouslycorresponding to the first stage starting from the top of the column 35.

The treated gas 15 contains in this example 1.36% molar of nitrogen,96.80% molar of methane and 1.76% molar of C₂ hydrocarbons.

More generally, the treated gas 15 contains more than 99% molar of themethane contained in the initial natural gas stream 13 and less than0.1% molar of the C₃ ⁺ hydrocarbons contained in the initial natural gasstream.

The treated gas 15 contains a molar proportion varying between 2% and85% of the C₂ hydrocarbons contained in the initial natural gas stream13, this proportion being adjustable.

The gas 15 thus comprises a content of C₆ ⁺ hydrocarbons of less than 1ppm, a water content of less than 1 ppm, advantageously less than 0.1ppm and a carbon dioxide content of less than 50 ppm. The treated gas 15may therefore be directly sent to a liquefaction train in order toproduce liquefied natural gas. It may also be directly sent to a gasdistribution network.

In the side reboiling circuits 37 and 39, side reboiling streams 161 and163 are extracted from the column 35 and are reintroduced therein afterbeing heated up in the first upstream heat exchanger 25, by heatexchange with at least one portion of the initial natural gas stream 13and at least one portion of the recycling stream 152.

Thus, an upper side reboiling stream 163 is sampled at a level N6located under level N1, for example at the eleventh stage starting fromthe top of the column 35, and is then brought as far as the first heatexchanger 25. The stream 163 is then heated up in the exchanger 25 andthen sent back into the column 35 at a level N7 located under the levelN6.

Also, a lower side reboiling stream 161 is sampled at a level N8 locatedunder the level N7, and is then brought into the heat exchanger 25. Thestream 161 is then heated up in the heat exchanger 25 and is thenreintroduced at a level N9 located under the level N8, for example atthe fourteenth stage starting from the top of the column 35.

In the bottom reboiling circuit 41, a liquid bottom reboiling stream 165is extracted in the vicinity of the foot of the column 35, below theside reboiling streams 161, 163.

According to the invention, the stream 165 is brought into the firstupstream heat exchanger 25 where it is heated up by heat exchange withat least one portion of the initial natural gas stream 13 and at leastone portion of the recycling stream 152. The heated up and partlyvaporized bottom reboiling stream is then reintroduced into the column35.

A bottom stream 171 rich in C₂ ⁺ hydrocarbons is extracted from the footof the recovery column 35.

The bottom stream 171 contains more than 99% molar of C₃ ⁺ hydrocarbonscontained in the initial natural gas stream 13. It has a methane contentcomprised between 9% and 5%.

The bottom stream 171 is pumped with the tank bottom pump 47 andintroduced at an intermediate level P1 of the fractionation column 61.

In the illustrated example, the fractionation column 61 operates at apressure comprised between 20 and 42 bars. In this example, the pressureof the fractionation column 61 is greater by at least one bar than thepressure of the recovery column 35.

A foot stream 181 is extracted from the fractionation column 61 in orderto form the cut 17 of C₃ ⁺ hydrocarbons.

The extraction rate of the C₃ ⁺ hydrocarbons in the method is greaterthan 99%. In every case, the propane extraction rate is greater than99%.

The ethane-rich stream 19 is directly drawn off at an intermediate levelP2 located in the upper region of the fractionation column 61.

In the example illustrated in the Figures, this stream comprises 1.21%of methane, 97.77% of ethane and 1.00% of propane.

More generally, the molar ethane content in the ethane-rich stream 19 isgreater than 95%, notably comprised between 96% and 100%.

The number of theoretical plates between the head of the column 61 andthe upper level P2 is for example comprised between 1 and 7. The levelP2 is above the feed level P1.

A second head stream 183 is extracted from the head of the column 61 andis then cooled in the second cooler 67 in order to form a second cooledand at least partly condensed head stream 185. This second stream 185 isintroduced into the second separator flask 69 for producing a liquidfraction 187 and a gas fraction 188.

In the example illustrated in FIG. 1, the totality of the liquidfraction 187 is pumped in the pump 71 in order to form a primary refluxstream 190 before being reintroduced with reflux into the fractionationcolumn 61 at a head level P3 located above the level P2.

In this case, the totality of the gas fraction 188 forms, after coolingin the head heat exchanger 33 and expansion in a valve 193, a secondaryreflux stream 192.

In the head exchanger 33, the gas fraction 188 is cooled by heatexchange with the head stream 131.

In an alternative illustrated in dotted lines, the liquid fraction 187is separated into a liquid primary reflux fraction 189 and a liquidsecondary fraction 191.

The secondary liquid fraction 191, when it is present, is then mixedwith the gas fraction 188 in order to form after cooling and expansion,the secondary reflux stream 192.

The secondary reflux stream 192 is introduced with reflux at a headlevel N4 of the recovery column 35 located between the head level N5 andthe intermediate level N3.

The ethane extraction rate, and subsequently the ethane flow rateproduced in the facility 11, is controlled by adjusting the flow rate ofthe recycling stream 152, by adjusting the pressure in the recoverycolumn 35, by means of the compressors 43 and 31 which are of thevariable rate type on the one hand, and by finally adjusting the flowrate of the secondary reflux stream 192 circulating through theexpansion valve 193 on the other hand.

As shown in the table below, the flow rate of the ethane-rich stream isadjustable, practically without affecting the extraction rate of C₃ ⁺hydrocarbons.

The method according to the invention therefore gives the possibility,with simple and inexpensive means, of obtaining a variable and easilyadjustable flow rate of an ethane-rich stream 19 extracted from theinitial natural gas 13, by maintaining the extraction rate of propaneabove 99%. This result is obtained without any significant modificationof the facility in which the method is applied.

Stream 152 Ethane Propane Total Pressure flow rate recovery recoverycompression C1 (bara) (kmol/h) (% by moles) (% by moles) power (kW) 29.00.37 0.66 99.76 16254 26.2 1900 15.00 99.48 17622 25.4 2600 29.34 99.0619072 24.8 4410 43.42 99.87 21389 22.5 5470 58.34 100 25861 20.7 575068.89 100 29554 19.1 6000 77.88 100 33136 17.9 6200 84.63 100 36183

The values of the pressures, the temperatures and flow rates in the casewhen the ethane recovery rate is equal to 84.99% are given in the tablebelow.

Stream Temperature (° C.) Pressure (bar abs) Flow rate (kmol/h) 13 20.050.0 38000 15 40.0 50.0 33634 17 86.8 33.5 978 19 11.9 33.0 3389 113−44.0 49.8 38000 115 −44.0 49.8 36412 120 −69.5 17.8 1588 125 −81.0 17.830858 128 −108.5 17.8 5554 131 −101.6 17.6 38134 152 40.0 50.0 4500 154−40.0 49.8 4500 155 −111.7 17.8 4500 171 −5.3 17.8 4376 192 −3.4 33.0 10194 −99.0 17.8 10

When the flow rate of the ethane-rich stream 19 is reduced, the totalcompression power is also strongly reduced.

The facility 11 according to the invention moreover does not require theimperative use of multiflow exchangers. It is thus possible to only useexchangers with tubes and a shell.

The treated natural gas 15 includes substantially nil contents of C₅ ⁺hydrocarbons, for example less than 1 ppm. Subsequently, if the carbondioxide content in the treated gas 15 is less than 50 ppm, this gas 15may be liquefied without any additional treatment or fractionation.

In the first method according to the invention, the bottom reboilingstream 165 is put into a heat exchange relationship in the first heatexchanger 25 with the recycling stream 152, with at least one portion ofthe head stream 131, with the initial natural gas stream 13 and with theside reboiling streams 161, 163.

This particular thermal integration of the method is beneficial in termsof yield, and does not affect the recovery of ethane, when the latter isdesired.

Thus, when the recycling stream 152 is placed in a heat exchangerelationship with at least one portion of the head stream 131, and whenthe side reboiling stream 165 is placed in a heat exchange relationshipwith the initial natural gas stream 13, the inventors surprisinglynoticed a synergistic increase in the yield of the facility 11.

Thus, as illustrated in the table below, a 16% yield gain is observed ascompared with the facility according to the state of the art whilepreserving a recovery rate of 85%, all the other conditions beingmaintained. This extremely significant gain is obtained, whilemaintaining very high ethane recovery.

Ethane recovery Total Case (% by moles) power (kW) Gain (%) State of theart 85.01 44756 — U.S. Pat. No. 7,458,232 Facility 11 85.00 40566 9.4without recycling of treated gas Facility 11 85.04 44651 0.2 without anyintegrated bottom reboiler Facility 11 84.99 37422 16.4

Moreover, the combined presence of the recycling of a portion of theheated gas and of an integrated bottom reboiling assembly 41 integratedinto the first heat exchanger 25 surprisingly generates a larger yieldgain than what is observed in the presence of either one of these stepstaken individually.

Thus, when the first method is applied without any treated gas recyclingstream 152, the obtained gain is 9.4%, while, when the first method 11is applied without a bottom reboiler integrated into the heat exchanger25, the obtained gain is 0.2%. The observed gain by sharing theaforementioned features is therefore notably greater than the sum of theindividual gains obtained, demonstrating an unexpected synergisticeffect, which does not affect ethane recovery.

Alternatively, the treated gas stream stemming from the first compressor31 may be brought into a compressor 43 with two equivalent power stages,with an intermediate cooler cooling the gas to the same temperature asthe cooler 45.

A second facility 201 according to the invention is illustrated by FIG.2. The facility 201 differs from the first facility 11 in that itfurther includes an auxiliary expansion turbine 203 and an auxiliarycompressor 205 coupled with a turbine 203. In a first embodiment, theauxiliary compressor 205 is interposed between the first compressor 31and the second compressor 43.

A second method according to the invention is applied in the secondfacility 201.

Unlike the first method according to the invention, the initial naturalgas stream 13 is separated into a first initial stream 207 and a secondinitial stream 209.

The molar flow rate of the first initial stream 207 is advantageouslygreater than the molar flow rate of the second initial stream 209.

Next, the first initial stream 207 is introduced into the first heatexchanger 25 so as to be cooled and partly condensed therein and to formthe cooled natural gas stream 113 introduced into the first separatorflask 27.

The second initial stream 209 is introduced into the auxiliary expansionturbine 203, so as to be expanded therein down to a pressure close tothe operating pressure of the column 35 and to form an auxiliary refluxstream 211. The auxiliary reflux stream 211 is then introduced into thefirst head heat exchanger 33 so as to be cooled and partly condensedtherein, and then into an expansion valve 213 for forming an expandedauxiliary reflux stream 215.

The stream 215 is then introduced into the recovery column 35 at anupper level N10 located between the level N3 and the level N4.

In the example illustrated in FIG. 2, the head stream 217 stemming fromthe first compressor 31 is introduced, at its outflow from the firstcompressor 31, into the auxiliary compressor 205, so as to be compressedat an intermediate pressure, before joining up with the secondcompressor 43.

The values of the pressures, temperatures, and flow rates in the casewhen the ethane recovery rate is equal to 85.00% are given in the tablebelow.

Stream Temperature (° C.) Pressure (bar abs) Flow rate (kmol/h) 13 20.050.0 38000 15 40.0 50.0 33634 17 87.7 34.0 978 19 12.6 33.5 3389 113−50.1 49.8 35074 115 −50.1 49.8 31965 120 −79.3 16.5 3109 125 −88.8 16.529505 128 −110.9 16.5 2460 131 −102.9 16.3 36154 152 40.0 50.0 2520 154−50.0 49.8 2520 155 −113.5 16.5 2520 171 −8.4 16.5 4376 192 −2.0 33.5 10194 −100.3 16.5 10 207 20.0 50.0 35074 211 −26.3 20.9 2926 215 −107.016.5 2926

The application of the second method according to the invention producesa result similar to that of the first method, by the synergy observedbetween the establishment of a heat exchange relationship of the bottomreboiling stream 165 with the initial natural gas stream 13, taken as acombination with the presence of a recycling stream 152, put into a heatexchange relationship with at least one portion of the head stream 131.

Thus, the consumption of the method for applying the facility 201 leadsto a consumed power equal to 37,588 kW, i.e. a gain of 16% as comparedwith the facility of the state of the art.

In an alternative of FIG. 2 (visible as dotted lines), the auxiliarycompressor 205 is mounted downstream from the compressor 43 in order tocompress the recycling stream 152, before introducing it into the firstheat exchanger 25.

The facility and the application of the method are moreover similar tothe one of FIG. 2.

A third facility 221 according to the invention is illustrated by FIG.3. Unlike the facility 11 illustrated in FIG. 1, the facility 221includes a second upstream separator flask 223 placed downstream fromthe first separator flask in order to collect the liquid phase 117stemming from the first separator flask 27.

A third method according to the invention is applied by means of thefacility 221. This third method differs from the first method accordingto the invention, in that the liquid phase 117 is expanded in a staticexpansion valve 225. This expansion is carried out down to a pressureabove the operating pressure of the column 35.

The liquid phase is then expanded and introduced into the upstreamseparator flask 223.

A liquid fraction 227 is recovered at the bottom of the flask 223 and isexpanded in a valve 229 in order to form an expanded fraction 231. Theexpanded fraction 231 is introduced into the recovery column 35 at levelN1.

A gas fraction 233 is collected at the head of the second upstreamseparator flask 223. This fraction 233 is sent towards the head heatexchanger 33 so as to be cooled therein before being expanded in anexpansion valve 135 in order to form an expanded fraction 237.

The expanded fraction 237 is introduced into the recovery column 35 atan intermediate level N11 comprised between the level N2 and the levelN3.

The values of the pressures, temperatures and flow rates in the casewhen the ethane recovery rate is equal to 84.99% are given in the tablebelow:

Stream Temperature (° C.) Pressure (bar abs) Flow rate (kmol/h) 13 20.050.0 38000 15 40.0 50.0 33658 17 86.8 33.5 978 19 13.1 33.0 3364 113−42.7 49.8 38000 115 −42.7 49.8 36709 117 −42.7 49.8 1291 118 −62.3 23.31291 125 −79.4 18.0 32325 128 −108.1 18.0 4384 131 −101.4 17.8 39758 15240.0 50.0 6100 154 −40.0 49.8 6100 155 −111.3 18.0 6100 171 −3.5 18.04392 188 7.2 33.0 50 192 −98.8 18.0 50 231 −67.4 18.0 910 233 −62.3 23.3381 237 −106.2 18.0 381

The method applied by means of a third facility 221 according to theinvention leads to a total power consumed by the compressors of 35,960kW, i.e. a gain of 19.7% relatively to the method of the state of theart.

It further allows an additional gain of 3.9% as compared with the firstmethod according to the invention.

In an alternative of the third method, the liquid phase 117 obtained atthe foot of the first separator flask 27 is introduced into the firstheat exchanger 25 so as to heat it up therein, before being brought intothe valve 225.

The mixture is expanded in the valve 225, before being separated in thesecond upstream separator flask 223.

A fourth facility 241 according to the invention is illustrated by FIG.4. Unlike the first facility 11, the stream 171 stemming from therecovery column 35 is passed into the first heat exchanger 25 so as tobe heated up therein before being introduced into the fractionationcolumn 61.

The fourth method according to the invention therefore applies heatingup of this bottom stream 171, after its passing into the pump 47.

For an ethane recovery rate of 85.00%, the total consumption is then of34,201 kW, which provides a gain of 23.6% as compared with the facilityof the state of the art. The gain is moreover 8.6% relatively to thefirst method according to the invention.

The values of the pressures, temperatures and flow rates in the casewhen the ethane recovery rate is equal to 85.00% are given in the tablebelow:

Stream Temperature (° C.) Pressure (bar abs) Flow rate (kmol/h) 13 20.050.0 38000 15 40.0 50.0 33656 17 86.8 33.5 976 19 12.9 33.0 3368 113−40.1 49.8 38000 115 −40.1 49.8 37218 120 −65.8 16.2 782 125 −80.1 16.227578 128 −110.6 16.2 9640 131 −102.9 16.0 34051 152 40.0 50.0 395 154−40.0 49.8 395 155 −113.9 16.2 395 171 −7.7 16.2 4354 188 5.4 33.0 10192 −100.2 16.2 10 243 12.0 33.5 4354

A fifth facility according to the invention 251 is illustrated by FIG.5. This facility is intended to apply a fifth method according to theinvention.

Unlike the first method according to the invention, a bypass stream 253is sampled in the recycling stream 152, advantageously downstream fromthe first heat exchanger 25 and upstream from the second heat exchanger33, so as to be reintroduced into the stream located downstream from thefirst dynamic expansion turbine 29.

The bypass stream flow rate 253 is for example equal to 47% of the totalmolar flow rate of the recycling stream 152 sampled in the treatedstream.

The fifth method according to the invention is moreover appliedsimilarly to the fourth method according to the invention.

In the example of FIG. 5, the bypass stream 253 is mixed with the feedstream 121 before it is introduced into the turbine 29.

In an alternative illustrated in dotted lines, the fifth facility 251further includes a secondary dynamic expansion turbine 255 coupled witha secondary compressor 257. A secondary recycling stream 258 is thensampled in the recycling stream 152 before its introduction into thefirst heat exchanger 25.

The secondary recycling stream 258 is introduced into the secondaryexpansion turbine 255, in order to form an expanded secondary recyclingstream 261, which is reintroduced into the partly heated-up head stream139 stemming from the first head heat exchanger 33.

Moreover, a secondary head stream 263 is sampled in the heated-up headstream 140 stemming from the first heat exchanger 25 so as to be broughtas far as the secondary compressor 257 and form a compressed secondaryhead stream 265.

This stream 265 is then reintroduced into the compressed head stream atan intermediate pressure, stemming from the first compressor 31 upstreamfrom the second compressor 43.

The power gain obtained relatively to the method of the state of the artis then of the order of 15.4%, for a total consumed power of 37,851 kW.

The values of the pressures, temperatures and flow rates in the casewhen the ethane recovery rate is equal to 85.00% are given in the tablebelow:

Stream Temperature (° C.) Pressure (bar abs) Flow rate (kmol/h) 13 20.050.0 38000 15 40.0 50.0 33633 17 86.8 33.5 978 19 11.9 33.0 3389 113−47.4 49.8 38000 115 −47.4 49.8 35524 120 −74.1 17.7 2477 125 −84.8 17.731199 128 −108.8 17.7 6463 131 −101.7 17.5 38183 152 40.0 50.0 4550 154−40.0 49.8 4550 155 −111.8 17.7 2412 171 −5.5 17.7 4377 188 −3.4 33.0 10192 −99.1 17.7 10 253 −40.0 49.8 2138

A sixth facility 271 according to the invention is illustrated in FIG.6. This facility 271 is intended for de-bottlenecking a facility asillustrated in U.S. Pat. No. 7,458,232 and initially comprising a firstupstream heat exchanger 25, a first separator flask 27, a recoverycolumn 35, a first head heat exchanger 33 and a fractionation column 61provided with a head condenser 63.

Unlike the first facility 11 according to the invention, the facility271 further includes a second upstream heat exchanger 273 and a thirdupstream heat exchanger 275, intended to be placed in parallel with thefirst upstream heat exchanger 25.

The facility 271 further includes an auxiliary compressor 277 intendedto compress the recycling stream 152 and an auxiliary cooler 279intended to cool the compressed recycling stream.

Moreover, the sixth facility 271 includes a second head heat exchanger281 intended to be placed in parallel with the first head heat exchanger33, in order to place at least one portion of the head stream 131 in aheat exchange relationship with at least one portion of the recyclingstream 152.

A sixth method according to the invention is applied in the sixthfacility 271. In this method, the initial natural gas stream 13 isseparated into a first initial stream 207 introduced into the firstupstream heat exchanger 25 and into a second initial stream 209introduced into a second upstream heat exchanger 273.

The first initial stream 207 is then cooled in the first upstream heatexchanger 25 in order to form a first cooled initial stream 281A. Also,the second initial stream 209 is cooled in the second upstream heatexchanger 273 in order to form a second cooled initial stream 283. Thestreams 281A and 283 are mixed so as to form the cooled stream 113intended to be introduced into the first upstream separator flask 27.

The side reboiling streams 161, 163 are introduced into the first heatexchanger 25 in order to be heated up therein.

Unlike the first method according to the invention, the bottom reboilingstream 165 is introduced into the second upstream heat exchanger 273 soas to be heated up therein by heat exchange with the second initialstream 209.

Also, unlike the first method according to the invention, the headstream 131 stemming from the recovery column 35 is first of allseparated into a first head stream fraction 285 and a second head streamfraction 287.

The first fraction 285 is introduced into the first head heat exchanger33 so as to be heated up therein by heat exchange with the reflux stream123 on the one hand and with the secondary reflux stream 192 on theother hand.

The second fraction 287 is introduced into the second head heatexchanger 281.

The ratio of the molar flow rate of the first fraction 285 to the secondfraction 287 is for example comprised between 0 and 20.

Next, the fractions recovered at the outlet of the heat exchangers 33,281 are mixed again before being again separated into a first portion289 of the heated-up head stream and into a second portion 291 of theheated-up head stream.

The first portion 289 is introduced into the first upstream heatexchanger 25 so as to be heated up therein by heat exchange with thefirst initial stream 207, simultaneously with the side reboiling streams161 and 163.

The second portion 291 is introduced into the third upstream heatexchanger 275 so as to be heated up therein.

The heated-up portions 289 and 291 are then joined together in order toform the heated-up head stream 140 and are then brought to the firstcompressor 31.

Unlike the first method according to the invention, the recycling stream152 is sampled in the heated head stream 140 upstream from the firstcompressor 31.

The ratio of the molar flow rate of the recycling stream 152 to themolar flow rate of the head stream 131 stemming from the column 35 isfor example comprised between 0% and 25%.

The recycling stream 152 is then compressed in the auxiliary compressor277, up to a pressure for example greater than 50 bars, and is thencooled in the cooler 279 in order to form a cooled compressed recyclingstream 293.

The stream 293 is then successively introduced into the third upstreamheat exchanger 275, and then into the second head heat exchanger 281 soas to be cooled therein, before being expanded in an expansion valve 295and to form a cooled expanded recycling stream 297.

The stream 297 is then introduced into the recovery column 35, at thesame level as the secondary reflux stream 194.

Thus, in the first upstream heat exchanger 25 initially present in thefacility, a portion 207 of the initial natural gas stream 13, the sidereboiling streams 161, 163 and a portion 289 of the head stream areplaced in a heat exchange relationship.

In the second upstream heat exchanger 273, a second portion 209 of theinitial natural gas stream 13, and the bottom reboiling stream 165 areplaced in a heat exchange relationship. In the third upstream heatexchanger 275, a second portion 291 of the head stream 131, and therecycling stream 152 are placed in a heat exchange relationship.

The facility 271 according to the invention moreover does not requireany imperative use of multiflow exchangers. It is thus possible to onlyuse exchangers with tubes and a shell.

Further at the head of the column 35, the reflux stream 123, a firstportion of the head stream 285, and the secondary reflux stream 192 areplaced in a heat exchange relationship in the first head heat exchanger33. In the second head heat exchanger 281, a second portion 287 of thehead stream 131 and the cooled compressed recycling stream 233 areplaced in a heat exchange relationship.

The facility 271 as illustrated in FIG. 6 gives the possibility ofaccommodating increases in the feed flow rate from 0% to 15% and morepreferentially of at least 10%, by limiting to a minimum the requiredincrease in compression power.

The values of the pressures, temperatures, and flow rates in the casewhen the ethane recovery rate is equal to 85.00% are given in the tablebelow:

Stream Temperature (° C.) Pressure (bar abs) Flow rate (kmol/h) 13 20.050.0 39900 15 40.0 50.0 35336 17 90.1 33.5 956 19 13.4 33.0 3608 113−44.3 49.8 39900 115 −44.3 49.8 38154 120 −74.8 13.5 1746 125 −89.1 13.529961 128 −115.6 13.5 8193 131 −106.5 13.3 36016 140 15.9 12.9 36016 141150.2 50.2 35336 152 15.9 12.9 680 171 −15.0 13.5 4565 192 5.0 33.0 1194 −103.7 13.5 1 207 20.0 50.0 39900 225 −118.4 13.5 680 285 −106.513.3 33250 287 −106.5 13.3 2766 289 −58.6 13.1 34935 291 −58.6 13.1 1080293 40.0 50.2 680

In the example illustrated in the figures, the ethane-rich stream 19 isdirectly sampled in the fractionation column 61, advantageously at anupper level P2 of the column 61 defined above.

The cut of C₃ ⁺ hydrocarbons 17 is moreover directly formed by the footstream 181 of the column 61.

In an alternative (not shown), the C₂ ⁺ hydrocarbons are extracted fromthe fractionation column 61 with the foot stream 181, at the same timeas the C₃ ⁺ hydrocarbons. The foot stream 181 is then introduced into adownstream fractionation column.

The ethane-rich cut 19 like the cut of C₃ ⁺ hydrocarbons 17 are thenproduced in the downstream fractionation column.

1. A method for simultaneously producing a treated natural gas, a cutrich in C₃ ⁺ hydrocarbons and under at least certain productionconditions, an ethane-rich stream, from an initial natural gas streamcontaining methane, ethane and C₃ ⁺ hydrocarbons, the method comprisingthe following steps: cooling and partly condensing the initial naturalgas stream in at least one first upstream heat exchanger in order toform a cooled initial stream; separating the cooled initial gas streaminto a liquid flow and a gas flow; expanding the liquid flow, andintroducing a stream stemming from the liquid flow into a column forrecovering C₂ ⁺ hydrocarbons at a first intermediate level; forming astream for feeding a turbine from the gas flow; expanding the feedstream in a dynamic expansion turbine and introducing it the expandedfeed stream into the recovery column at a second intermediate level;recovering and compressing at least one portion of the head stream ofthe recovery column in order to form the natural gas and recovering thefoot stream of the recovery column in order to form a C₂⁺-hydrocarbon-rich liquid stream; introducing the liquid stream at afeed level (P1) of a fractionation column provided with a headcondenser, the ethane-rich stream being produced under said productionconditions, from a stream stemming from the fractionation column, thefractionation column producing a foot stream intended to at least partlyform the C₃ ⁺ hydrocarbon cut; introducing a primary reflux streamproduced in the head condenser with reflux into the fractionationcolumn; producing a secondary reflux stream from the head condenser andintroducing the secondary reflux stream at the head of the recoverycolumn, sampling a recycling stream in the head stream stemming from therecovery column; establishing a heat exchange relationship of therecycling stream with at least one portion of the head stream stemmingfrom the recovery column, reintroducing, after expansion, the cooled andexpanded recycling stream into the recovery column, sampling in thebottom of the recovery column of at least one bottom reboiling stream,establishing a heat exchange relationship of the bottom reboiling streamwith at least one portion of the initial natural gas or/and with therecycling stream, ensuring the bottom reboiling by the calories takenfrom the initial natural gas stream or/and from the recycling stream. 2.The method according to claim 1, including placing at least one portionof the head stream of the recovery column and the recycling stream in aheat exchange relationship with the initial natural gas stream and withthe bottom reboiling stream.
 3. The method according to claim 1,including putting in a heat exchange relationship in a first head heatexchanger the recycling stream stemming from the first upstream heatexchanger, the secondary reflux stream stemming from the head condenser,and the head stream stemming from the recovery column.
 4. The methodaccording to claim 1, including sampling at least one side reboilingstream above the bottom reboiling stream, and placing said or each sidereboiling stream in a heat exchange relationship with at least oneportion of the initial natural gas stream.
 5. The method according toclaim 1, including drawing off the ethane-rich stream from anintermediate level of the fractionation column located above the feedlevel of the column, and below the head level of the fractionationcolumn.
 6. The method according to claim 1, including the followingsteps: separating the initial natural gas stream into a first initialstream and into a second initial stream; introducing the first initialstream into the first upstream heat exchanger; introducing at least oneportion of the second initial stream into an auxiliary dynamic expansionturbine in order to form an auxiliary reflux stream from the effluentstemming from the auxiliary turbine; introducing the auxiliary refluxstream into the recovery column.
 7. The method according to claim 6,including at least one portion of the recycling stream in an auxiliarycompressor coupled with the auxiliary turbine.
 8. The method accordingto claim 1, including compressing at least one portion of the headstream in an auxiliary compressor coupled with the auxiliary turbine. 9.The method according to claim 1, including sampling, in the recyclingstream, bypass stream, and reintroducing the bypass stream into a streamlocated upstream from the first dynamic expansion turbine (29).
 10. Themethod according to claim 1, including expanding the liquid flowstemming from the first upstream separator flask and introducing saidliquid flow into a second upstream separator flask in order to form aliquid fraction and a gas fraction, the liquid fraction being introducedafter expansion at the first intermediate level of the recovery column,the gas fraction being introduced at an upper level of the recoverycolumn, located above the intermediate level.
 11. The method accordingto claim 1, including establishing a heat exchange relationship of thefoot stream stemming from the recovery column with the initial naturalgas stream and with the bottom reboiling stream in the first upstreamheat exchanger before its introduction into the fractionation column.12. The method according to claim 1, including separating the gas flowstemming from the first separator flask into the feed stream and into areflux stream, the feed stream being intended to feed the dynamicexpansion turbine, and introducing the reflux stream being introduced,after cooling, partial or total condensation, and expansion in a valve,with reflux, into the recovery column.
 13. A facility for simultaneouslyproducing a treated natural gas, a cut rich in C₃ ⁺ hydrocarbons, andunder certain production conditions, an ethane-rich stream, from aninitial natural gas stream containing methane, ethane and C₃ ⁺hydrocarbons, the facility comprising: an assembly for cooling andpartly condensing the initial natural gas stream comprising at least aone first upstream heat exchanger in order to form a cooled initialstream; an assembly for separating the cooled initial stream into aliquid flow and into a gas flow; a column for recovering C₂ ⁺hydrocarbons an assembly for expanding the liquid flow, and forintroducing a stream stemming from the liquid flow into the recoverycolumn at a first intermediate level; an assembly for forming a streamfor feeding the turbine from the gas flow; an assembly for expanding thefeed stream, comprising a dynamic expansion turbine and an assembly forintroducing the expanded feed stream into the recovery column at asecond intermediate level; an assembly for recovering and compressing atleast one portion of the head stream of the recovery column in order toform the natural gas and an assembly for recovering the foot stream ofthe recovery column in order to form a liquid stream rich in C₂ ⁺hydrocarbons; a fractionation column provided with a head condenser, anassembly for introducing the liquid stream at a feed level of thefractionation column, the ethane-rich stream being able to be producedunder said production conditions, from a stream stemming from thefractionation column, the fractionation column being able to produce afoot stream intended to form at least partly the C₃ ⁺ hydrocarbon cut;an assembly for introducing a primary reflux stream produced in the headcondenser with reflux into the fractionation column; an assembly forproducing a secondary reflux stream from the head condenser and anassembly for introducing the secondary reflux stream at the head of therecovery column, an assembly for sampling a recycling stream in the headstream of the recovery column; an assembly for establishing a heatexchange relationship of the recycling stream with at least one portionof the head stream stemming from the recovery column, an assembly forreintroducing, after expansion, the recycling stream into the recoverycolumn, an assembly for sampling in the bottom of the recovery column atleast one bottom reboiling stream, and an assembly for establishing aheat exchange relationship of the bottom reboiling stream with at leastone portion of the initial natural gas or/and with the recycling stream,the reboiling being able to be ensured by the calories taken from theinitial natural gas stream or/and from the recycling stream.
 14. Thefacility according to claim 13, including a first upstream heatexchanger capable of establishing a heat exchange relationship with atleast one portion of the initial natural gas stream, the bottomreboiling stream, at least one portion of the head stream and therecycling stream.
 15. The facility according to claim 13, including afirst upstream heat exchanger capable of establishing a heat exchangerelationship of a first portion of the initial natural gas stream, withat least one portion of the head stream, a second upstream heatexchanger, distinct from the first upstream heat exchanger, capable ofestablishing a heat exchange relationship of a second portion of theinitial gas stream with the bottom reboiling stream stemming from therecovery column, and a third upstream heat exchanger distinct from thefirst upstream heat exchanger and from the second upstream heatexchanger, the third upstream heat exchanger being capable ofestablishing a heat exchange relationship of at least one portion of therecycling stream with at least one portion of the head stream, includingan auxiliary compressor capable of compressing the portion of therecycling stream intended to be introduced into the third upstream heatexchanger.
 16. The method according to claim 8, wherein the auxiliarycompressor is coupled with the auxiliary turbine between a firstcompressor coupled with the first turbine and a second compressor. 17.The method according to claim 10, including placing the liquid flowstemming from the first upstream separator flask in a heat exchangerelationship with the initial natural gas stream so as to be heated upbefore being introduced into the second upstream separator flask. 18.The facility according to claim 14, wherein the first upstream heatexchanger is capable of establishing a heat exchange relationship withside reboiling streams.
 19. The facility according to claim 15,including an auxiliary compressor capable of compressing the portion ofthe recycling stream intended to be introduced into the third upstreamheat exchanger.